As a background technique in this related art, there is disclosed JP 10-73068 A (PTL 1). In the publication, there is disclosed “an ignition timing control device of an internal combustion engine equipped with an ignition timing control means to control an ignition timing of the internal combustion engine, comprising: an air-fuel-ratio detection means which is provided in an exhaust system of the engine; and an air-fuel ratio estimation means which sets an observer to observe an inner state on the basis of a model describing a behavior of the exhaust system of the engine and receives an output of the air-fuel-ratio detection means to estimate an air-fuel ratio of each cylinder, wherein the ignition timing control means controls the ignition timing of each cylinder of the engine according to an estimated air-fuel ratio of each cylinder.” (see [Claim 1]). In addition, there is disclosed JP 2010-242630 A (PTL 2). In the publication, there are disclosed “a control device of a multi-cylinder engine, comprising: a supplying air adjustment means that adjusts an air amount to be supplied into a cylinder; an EGR passage through which part of an exhaust is recirculated to an intake system; an EGR gas amount adjustment means which adjusts an EGR gas amount recirculating to the intake system through the EGR passage; a combustion state estimation means which detects or estimates a combustion state of an air-fuel mixture; an air-fuel-ratio detection means which detects an air-fuel ratio of the exhaust; and a control means which controls the supplying air adjustment means and the EGR gas amount adjustment means, wherein the control means performs the EGR at the time of a heavy load operation in which a load of the engine is equal to or more than a predetermined value, and obtains a variation degree of the combustion state between the cylinders and/or a variation degree of the air-fuel ratio between the cylinders on the basis of at least one of an estimation result of the combustion state estimation means and a detection result of the air-fuel-ratio detection means, and controls an EGR rate on the basis of the variation degree,” and “the control device of the multi-cylinder engine in which the control means controls also an ignition timing in addition to the EGR rate (see [Claim 2] and [Claim 3]).